INTERACTION AMONG MU-OPIOID RECEPTORS AND ALPHA-2-ADRENOCEPTORS ON SH-SY5Y HUMAN NEUROBLASTOMA-CELLS

The clonal human neuroblastoma cell line SK-N-SH-SY5Y was previously shown to express mu-opioid an alpha-2-adrenoceptors which are both negatively coupled to adenylyl cyclase. Because of the potential use of alpha-2-agonists in the treatment of narcotic dependence, we tested the interactions among he alpha-2-agonists, clonidine and norepinephrine, and morphine on AC in SH-SY5Y cells. Pretreatment with retinoic acid resulting in partial neuronal differentiation greatly enhanced the cells' sensitivity towards adenylyl cyclase stimulation by prostaglandin E1, and its inhibition by morphine and alpha-2-agonists. Norepinephrine (EC50 = 69 nM) maximally inhibited prostaglandin E1-stimulated cAMP accumulation (by approximately 83%), and the alpha-2-antagonist yohimbine reversed these effects. Clonidine (EC50 = 32 nM) was a partial agonist, with 50 to 60% maximal inhibition. The combined effects of morphine (maximum approximately 70% inhibition) and norepinephrine exceeded the effect of either agent alone, yielding more than 90% inhibition of prostaglandin E1-stimulated cAMP accumulation. As previously reported for morphine, only a partial tolerance was observed for adenylyl cyclase inhibition by norepinephrine. Further, no cross-tolerance was observed between clonidine and morphine. The combined results indicate that mu-opioid receptors and an alpha-2-adrenoceptor subtype are colocalized on the same cells in SH-SY5Y culture, which hence serves as a model to study opioid-alpha-2-adrenergic interactions.